1. Field of the Invention
The invention relates to vehicle speed control system and vehicle speed control method of a vehicle, such as an automobile, and more particularly to a vehicle speed control system that performs vehicle speed limit control and speed increase rate control and a vehicle speed control method.
2. Description of the Related Art
As one type of vehicle speed control system of a vehicle, such as an automobile, a vehicle speed control system is known which performs vehicle speed limit control for limiting the vehicle speed to the upper-limit vehicle speed or lower, for example, when an off-road vehicle descends a hill, and performs speed increase rate control for controlling the rate of increase of the vehicle speed to a target rate of speed increase when the vehicle speed limit control is finished.
For example, Japanese Patent Application Publication No. 2005-1632 (JP-A-2005-1632) discloses a vehicle speed control system that is arranged to perform constant low-speed running control under which the vehicle speed is limited to the upper-limit vehicle speed or lower, by inhibiting one or more gear position(s) of a transmission from being established, and cancel the inhibition of the gear positions from the lower-speed gear position one by one after the completion of the constant low-speed running control, thereby to prevent a rapid increase in the vehicle speed.
With the vehicle speed control system as described in the above-identified publication, the vehicle speed can be surely prevented from rapidly increasing after the constant low-speed running control is finished. However, the inhibition of the gear positions is cancelled according to a pattern that is determined based on the gear position of the transmission and the elapsed time at the time when the constant low-speed running control is finished, and the rate of increase of the vehicle speed is specified or determined by the cancellation of the inhibition of the gear positions. Therefore, a passenger or passengers of the vehicle may feel a sense of disagreement or feel uncomfortable about the speed increase, depending upon running conditions of the vehicle at the time when the constant low-speed running control is finished.
For example, even if the vehicle speed increases at the same rate after the constant low-speed running control is finished, the passenger of the vehicle may feel that the vehicle speed increases at a relatively high rate when the vehicle speed detected at the time of the completion of the constant low-speed running control is low, and may feel that the vehicle speed increases at a relatively low rate when the vehicle speed detected at the time of the completion of the constant low-speed running control is high. Also, the passenger of the vehicle may feel that the vehicle speed increases at a higher rate as the degree of inclination or gradient of a road on which the vehicle runs is larger during downhill running of the vehicle.